The present invention relates to a nickel-metal hydride battery, and more particularly, to a nickel-metal hydride battery that uses AB5 hydrogen-absorbing alloy for the negative electrode.
A nickel-metal hydride battery uses a hydrogen-absorbing alloy for the negative electrode instead of cadmium. In comparison with a battery using a cadmium electrode, the nickel-metal hydride battery increases the effective energy density relative to weight and volume. This increases the capacity of the battery, reduces toxicity, and is more environmentally friendly. However, the hydrogen-absorbing alloy used in a nickel-metal hydride battery is apt to corrode when incorporated in a battery since it is immersed in an electrolyte formed from a high-concentration alkaline solution. It is thus required that the hydrogen-absorbing alloy be resistant to alkaline corrosion. Further, the absorption and release of hydrogen during the charging and discharging of the battery causes volumetric expansion and contraction of the hydrogen-absorbing alloy and produces fine powder. This reduces the output and durability of the battery. Accordingly, it is significant that such fine powder of the hydrogen-absorbing alloy be suppressed to prolong the battery life. Especially in applications in which overcharging frequently occurs, lattice expansion of the alloy accelerates the production of fine powder.
An AB5 alloy is most commonly used as the hydrogen-absorbing alloy for a nickel-metal hydride battery. To suppress the production of fine powder, this alloy contains a large amount of cobalt (0.4 mol or more), which is expensive.
In the prior art, Japanese Laid-Open Patent Publication No. 9-45322 discloses a hydrogen-absorbing alloy that suppresses the production of fine powder when used for the negative electrode. The hydrogen-absorbing alloy has a CaCu5 type crystalline structure having a (111) plane with a peak half-width of 0.20 to 0.50 degrees when observed by performing powder X-ray diffraction. Liquid quenching is performed to form flakes or ribbons of the hydrogen-absorbing alloy. X-ray diffraction shows that 60% or more of the crystals in the hydrogen-absorbing alloy have a c-axis oriented in a direction orthogonal to the thicknesswise direction of the alloy.
The manufacturing method of the hydrogen-absorbing alloy proposed in the above publication is inefficient for mass production. Therefore, a large amount of AB5 alloy is manufactured through casting. However, in the AB5 alloy of the prior art, the contained amount of cobalt is substantially at least 0.4 mol, and the theoretical capacity of the negative electrode is at least 1.5 times greater than that of the positive electrode. This increases the usage amount of the hydrogen-absorbing alloy and the expensive cobalt and thereby raises manufacturing cost.